1. Field
The present invention relates to a transflective type liquid crystal display device, and more particularly, to a transflective type liquid crystal display device and method for manufacturing the same, capable of optimizing optical efficiency.
2. Description of the Related Art
Generally, liquid crystal display (LCD) devices have advantages such as being lightweight, having a slim profile, and low power consumption, and are widely used for portable computers, office automation equipment, and audio/video apparatuses.
The LCD device includes two substrates and a liquid crystal layer interposed between the two substrates, and displaces liquid crystal molecules using an electric field generated upon application of a voltage. Hence, an image is displayed by manipulating the transmission of light through the liquid crystal.
Since the LCD device does not generate light by itself, it uses ambient light or a backlight assembly for generating light. Generally, the LCD device can be classified into two different categories: a transmission type LCD device or a reflection type LCD device.
FIG. 1 is a cross-sectional view schematically showing a structure of the transmission type LCD device according to the related art. In FIG. 1, the transmission type LCD device includes: a first substrate 102 on which a thin film transistor (TFT) functioning as a switching element is formed on each of intersection points between a plurality of gate lines and data lines; a second substrate 101 which faces the first substrate 102 and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer 103 including liquid crystals interposed between the first and the second substrates 102 and 101; first and the second polarizing plates 105 and 104 arranged on an outer surface of each of the first and the second substrates 102 and 101; and a backlight assembly 106 disposed outside the first polarizing plate 105.
An optical transmission axis of the first polarizing plate 105 has an angle of 90° to that of the second polarizing plate 104. The backlight assembly 106 generates light and provides the light toward the first substrate 102.
In the related art LCD device having the foregoing construction, when the TFTs are turned on by a scanning signal applied to the plurality of gate lines and a data voltage applied to the plurality of data lines, the data voltage is applied to pixel electrodes through the turned-on TFTs. At this time, a common voltage is supplied to the common electrode of the second substrate 101. Accordingly, the liquid crystal molecules are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block light provided from the backlight assembly 106, so that a predetermined image is displayed.
However, in the transmission type LCD device of the related art, it is difficult to realize slimness and lightweight of the LCD device due to a large volume and a heavy weight of the backlight assembly 106. Also, the power consumption of the backlight assembly 106 increases the overall power consumption of the device by a significant amount.
Therefore, research into reflection type LCD devices using ambient light instead of the backlight assembly 106 is actively performed. Such a reflection type LCD device is widely used as a portable display device such as an electronic organizer and a PDA (Personal Digital Assistant) thanks to low power consumption.
FIG. 2 is a cross-sectional view schematically showing a structure of the reflection type LCD device according to the related art. In FIG. 2, the reflection type LCD device includes: a first substrate 202 on which a thin film transistor (TFT) functioning as a switching element is formed on each of crossing points between a plurality of gate lines and data lines; a second substrate 201 which faces the first substrate 202 and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer 203 including liquid crystals interposed between the first and the second substrates 202 and 201; a first and a second polarizing plates 205 and 204 arranged on an outer surface of each of the first and the second substrates 202 and 201; and a reflector 206 disposed outside the first polarizing plate 205.
An optical transmission axis of the first polarizing plate 205 has an angle of 90° to that of the second polarizing plate 204. The reflector 206 reflects light provided from the outside and provides the light toward the first substrate 202.
In the LCD device having the foregoing construction, when a plurality of TFTs are turned on by a scanning signal applied to a plurality of gate lines and a data signal applied to a plurality of data lines, the data signal is applied to pixel electrodes through the turned-on TFTs. At this time, a common voltage is supplied to the common electrode of the second substrate 201. Accordingly, the liquid crystals are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block light provided and reflected from the outside, whereby a predetermined image is displayed.
However, in the related art reflection type LCD device, when ambient light does not have a sufficient intensity (for example, the surrounding environment is dim), the brightness level of the display image is lowered and displayed information is not readable, which is problematic.
To resolve the above problems, a transflective type LCD device, which combines the reflection type LCD device and the transmission type LCD device, has been suggested.
FIG. 3 is a cross-sectional view schematically showing a construction of the transflective type LCD device according to the related art. In FIG. 3, the transflective type LCD device includes: a first substrate 330 on which a thin film transistor (TFT) functioning as a switching element is formed on each of crossing points between a plurality of gate lines and data lines; a second substrate 310, which faces the first substrate 330 and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer 320 including liquid crystals interposed between the first and the second substrates 330 and 310; a first and a second polarizing plates 331 and 311 arranged on a lower surface of the first substrate 330 and an upper surface of the second substrates 310, respectively; and a backlight assembly 340 disposed outside the first polarizing plate 331.
An optical transmission axis of the first polarizing plate 331 has an angle of 90° to that of the second polarizing plate 311.
On the first substrate 330, a pixel electrode is connected to each TFT. On the pixel electrodes, a passivation layer 322 having a transmission hole 321 exposing a portion (transmission region) of each of the pixel electrodes and a reflector 323 are sequentially formed.
It is assumed that a region corresponding to the reflector 323 is a reflection region ‘r’ and a region corresponding to the portion of the pixel electrode, exposed by the transmission hole 321, is a transmission region ‘t’. The reflection region ‘r’ is the region for reflecting light provided from ambient light in a reflection mode, and the transmission region ‘t’ is the region for transmitting light provided from the backlight assembly 340 in a transmission mode.
At this time, to reduce the difference in the distance that the light travels between the transmission region ‘t’ and the reflection region ‘r’, the cell gap d1 of the transmission region ‘t’ is about twice that of the cell gap d2 of the reflection region ‘r’.
Generally, a phase difference δ of a liquid crystal is obtained by the following formula:δ=Δn·d 
where δ is the phase difference of a liquid crystal, Δn is the refractive index of a liquid crystal, and d is the cell gap.
Therefore, a difference in optical efficiency is generated between the reflection mode and the transmission mode. To reduce this difference in optical efficiency, the cell gap d1 of the transmission region ‘t’ should be greater than the cell gap d2 of the reflection region ‘r’ such that the phase difference value of the liquid crystal layer 320 is constant.
However, even though the difference in optical efficiency is reduced by making the cell gap d1 of the transmission region t different from the cell gap d2 of the reflection region r, it is difficult to optimize the transmission region and the reflection region. Therefore, it is difficult to obtain optimized optical efficiency. For example, in the transmission mode, not all of the light provided from the backlight assembly is transmitted through the transmission region, and some of the light impinges on the reflection region and is not transmitted, whereby optical loss occurs. Also, in the reflection mode, not all the ambient light is reflected by the reflector, and some of the ambient light impinges on the backlight assembly through the transmission region, whereby optical loss occurs.